The present invention relates to controlling a semiconductor laser diode mounted to optical pickups of optical disc apparatuses.
A semiconductor laser diode (hereinafter referred to as SLD) emits light for recording and reproducing information. The SLD is mounted to an optical pickup of an optical disc apparatus. When reproducing information, the apparatus converges reproduced weak light onto a disc so that reflectance, phase difference and deflection angle of a pit recorded on the disc can be detected for reading the information. When recording information, the SLD emits light with a higher power than in reproducing, and the laser power is modulated responsive to the information to be recorded on the disc. When erasing information, the apparatus irradiates laser power, slightly weaker than recording power, onto the disc to erase the information.
For instance, when recording information into a phase change optical disc, the apparatus switches binary values of a laser power, i.e., basically, recording a mark by peak power, and erasing the recorded mark by bias power.
This switching of binary values of laser powers, however, is actually not enough to form a stable recorded mark. As shown in FIG. 1, switching a plurality of laser powers, namely more than two values, is required in order to uniform amounts of heat applied to marks to be recorded, and to adjust heat balance at a leading edge and an tailing edge of the mark.
Recently, the market has required a higher speed of recording and reproducing to/from optical discs, which entails a higher speed of switching the laser power.
When a recording pulse shown in FIG. 1 is tried to be realized in a high speed recording mode, laser power could not reach a desirable wave height shown in FIG. 2 in a case where a pulse waveform of a SLD was not sufficiently increased in speed. As a result, unbalanced heat is applied to a recorded mark, which distorts the recorded mark.
In the PWM (pulse width modulation) signal recording which has information at a leading and a tailing edges of a recorded mark, the edge position of the recorded mark needs to be accurately controlled. Therefore, a record pulse with shorter rising and falling times is required.
In a conventional system where an optical pickup including a SLD is separated from a driving device of the SLD, a driving current of the SLD is normally transmitted via flexible cable or the like. In this case, distributed constants of the flexible cable, such as parasitic capacity, degrade switching characteristics of the driving current. In other words, the degradation of switching characteristics obstructs the speedup of the optical disc apparatus.
The present invention aims to improve the disadvantage discussed above, and solve the resultant problem in due course, i.e. heat concentration, by saving power.
FIG. 3 shows a relation of a parasitic capacitance component of conductive bodies between the laser diode driving device and the SLD to Tr, Tf (rising time and falling time) of a SLD driving current when the SLD is switched pulse-wise.
FIG. 3 proves that Tr and Tf increase at a greater capacitance component, because the high frequency component of the transmitted pulse current is bypassed via the capacitance before reaching the SLD. The pulse waveform thus becomes dull as shown in FIG. 2.
Assume that a pulse of a maximum frequency xe2x80x9cf1xe2x80x9d is required to drive the SLD, a minimum pulse width should be 0.5/f1. Accordingly, a condition of no decrease in the amplitude of driving SLD is achieved by the following expression:
Tr less than 0.5xc3x970.5/f1, Tf less than 0.5xc3x970.5/f1xe2x80x83xe2x80x83Expression 1
For instance, to realize a pulse switching of 60 MHz=max. frequency, Expression 1 requires both of Tr and Tf be not more than 4.16 ns. The experiment result shown in FIG. 3 tells that a parasitic capacitance component should be not more than 10 pF in order to realize Tr, Tf less than 4.16 ns. To satisfy this condition, shortening the distance between the SLD and the laser diode driving device within 5 cm is required.
In order to realize a high speed laser diode current switching, the laser diode driving device and the SLD are desirably placed closer with each other so that the distributed constants therebetween, in particular, parasitic capacitance can be decreased. In other words, the laser diode driving device should be mounted on the optical pickup that incorporates the SLD, or the SLD driving device should be attached to a moving section where the optical pickup is disposed.
In this case, heat generated in the SLD and in the laser diode driving device causes a problem. During the recording mode onto a disc, SLD driving current xe2x80x9cIopxe2x80x9d reaches several hundreds mA, and a temperature rise Td in the driving device is calculated by the following equation:
Td=K1xc3x97Vdxc3x97Iopxe2x80x83xe2x80x83Expression 2
where:
Td=temperature rise in SLD driving device,
Vd=voltage across an output section of the driving device, and
K1=proportionality constant
Since the SLD driving device and the SLD should be placed closely with each other in order to decrease the influence of the distributed constants including the parasitic capacitance, the heat generated in the driving device travels to the SLD.
Driving characteristics of SLD largely depend on a temperature. The relation of light emission power of laser to the driving current xe2x80x9cIopxe2x80x9d degrades at higher temperature; i.e. differential efficiency is degraded. A greater driving current is thus required at a higher temperature, which accelerates temperature rise in the driving device according to Expression 1.
The SLD per se is a heat source, and its temperature rise xe2x80x9cTldxe2x80x9d is expressed by the following equation:
Tld=K2xc3x97Vopxc3x97Iopxe2x80x83xe2x80x83Expression 3
where:
Vop=operational voltage of the SLD,
K2=proportionality constant.
Assume that heat conductivity from the driving circuit to the SLD is set as K3, a temperature rise of the SLD is expressed by the following equation:
Tld=K2xc3x97Vopxc3x97Iop+K3xc3x97K1xc3x97Vdxc3x97Iopxe2x80x83xe2x80x83Expression 4
The heat problem discussed above may be solved by the following measures:
1. Restraining the driving current xe2x80x9cIopxe2x80x9d,
2. Restraining proportionality constants K1, K2 and heat conductivity K3, and
3. Restraining voltages Vd applied across the output section of the driving device and Vop.
xe2x80x9cIopxe2x80x9d is a characteristic proper to SLDs. Constants K1, K2 and K3 are dependent on constructions of optical pickups and heat mechanical designs. xe2x80x9cVopxe2x80x9d also follows the driving current xe2x80x9cIopxe2x80x9d. A change amount of xe2x80x9cVopxe2x80x9d is dependent on a diode voltage and inner series resistors, which is basically a value proper to SLDs. Therefore, the voltage xe2x80x9cVdxe2x80x9d across the output section of the SLD driving device is restrained so that the heat generation at the pickup can be restrained.
The following measures are taken to restrain xe2x80x9cVdxe2x80x9d: A power supply is provided so that a voltage supplied to the output section of the SLD driving device can be controlled, and the voltage at the output section can be monitored. The voltage is compared with a reference voltage, and the deference is fed back to the power supply so that xe2x80x9cVdxe2x80x9d can be always maintained at a constant level. Further, a voltage of the power supply is set at a minimum value within the driving range which still keeps the output section working.
The power supply discussed above is set at a place other than the optical pickup; i.e. a surplus heat source is isolated to a place where no thermal influence is effected to the SLD.
Further, even if drift of an ac line voltage, load drift or other variations would change the DC source voltage of the optical disc apparatus, the xe2x80x9cVdxe2x80x9d of the output section of the SLD driving device is kept at a constant level. The heat generated in the optical pickup thus is not affected by the changes of the line voltage of the apparatus.
As such, the voltage xe2x80x9cVdxe2x80x9d across the output section of the SLD driving device is kept at a lowest possible value for operation so that a thermal rising value in the pickup can be minimized. As a result, a high speed and stable SLD driving device can be realized.